A clutch or brake of this type, e.g. as used for speed changes in a planetary-gear train, comprises a cylinder with an annular working space in which an annular piston is axially slidable under fluid pressure toward a stack of interleaved annular friction plates, some of these plates being positively linked with the cylinder while the rest are linked with a central shaft or other relatively rotating member to be coupled with that cylinder. The fluid pressure driving the piston into its operating position is resisted by spring means tending to keep the piston separated from the stack.
For manufacture as well as for installation it is convenient to use cylinders of the same size for clutches or brakes operable under different coupling pressures. For a device of lower power it is merely necessary to use a piston having an effective width less than that of the working space whereby only a fraction of the available fluid pressure is transmitted to the stack. In such an instance, however, one must reduce the effective cross-sectional area of that space in order to prevent the leakage of fluid past the piston. For this purpose it has been customary to weld or otherwise secure a stationary ring either to the peripheral cylinder wall or to a central boss constituting the outer and inner boundaries of the working space.